Screen printing is a printing technique in which ink is squeezed through a patterned, perforated screen onto a substrate. Mainly it is used to print patterns on textiles but it is also used to print on plastics bottles and plastics material to produce signs. Another use is to print conductive ink patterns to produce wiring patterns on printed circuit boards and other devices
Screens typically are made of perforated cylindrical sleeves of nickel having a thickness of 0.1 mm, a diameter of between 200 and 400 mm and a length of 1 to 3 meters. Conventionally such screens are coated with a photosensitive lacquer which covers the outer surface of the screen and fills all of its perforations. To produce an ink transmitting pattern the lacquer is exposed to ultraviolet light through a transparent sheet carrying an opaque pattern which is usually a negative resulting from a photographic process. Portions of the photosensitive lacquer which are exposed to ultraviolet light remain after development of the lacquer, whilst the portions of the photosensitive lacquer which lie beneath the opaque regions are not exposed to the ultraviolet light and, during development are removed to open the perforations in the screen. This produces a pattern on the screen of areas which transmit ink and areas which do not transmit ink.
Recently it has been proposed that, instead of using a photographic process to produce a pattern of ink transmitting and non-transmitting regions on the screen they are patterned using a laser engraving device to ablate the lacquer from regions of the screen that are required to transmit ink. Such a technique is disclosed in DE-A-No. 3601327. This technique avoids the need to go through development and washing steps of the screen and enables the screen to be patterned directly from digital data without the need for an intermediate photographic negative. To do this it has been proposed to scan a laser engraving head over the entire surface of a screen with a laser beam being switched on during the time that it is scanning over a region in which ink is to be transmitted and switched off when scanning over a region where ink is not to be transmitted through the resulting screen. Such a laser beam engraving device effectively removes the lacquer and opens up the perforations. However, such techniques are quite slow since the laser beam is scanned over substantially the entire surface of the screen at a uniform speed as the laser beam is turned on and off as required to produce the pattern. Another problem is the generation of moire patterns particularly in half tone regions formed by ablating small, regularly spaced areas of lacquer from the surface of the screen. When the small regularly spaced areas are of a different pitch or arranged at a different angle to the perforations of the screen, a periodic overlap and non-overlap occurs between them and the perforations of the screen which gives rise to areas of high contrast across what is required to be a continuous half-tone region of the pattern. These contrast areas are resolvable by eye in the finished printed product.
In gravure printing a closely spaced array of cells are etched into the surface of the printing member and the depth of the individual cells controls the quantity of ink retained by each cell and hence the density of ink applied to a corresponding area of a printing substrate. Typically the size and spacing of such cells are not resolvable by eye and are very much more closely spaced than the perforations in a screen printing screen. In a prototype system for laser and electron beam engraving of rotogravure cylinders the metal cylinders are pre-engraved with an array of gravure cells which have been engraved to the maximum required depth and then filled with a plastics material such as an epoxy resin. The plastics material is then etched by the application of a laser or electron beam. GB-A-No. 1410344 describes a laser engraving system for engraving such rotogravure cylinders in which a scanning cell detector is located upstream of a laser or electron beam engraving head to detect the location of the engraved cells and a deflector is included in the laser or electron beam engraving head to deflect the beam in two mutually perpendicular directions as the engraving head is scanned over the surface of the printing member so that the beam impinges on the plastics material in the filled cells.
US-A-No. 4525823 describes an optical tracking system in which a reading head is controlled to follow a pre-recorded optical information track. A spot of light is projected onto the track and light reflected and scattered from it is focussed onto two side-by-side photodetectors, the outputs of which are compared and the result used to control the point of projection of the light spot. GB-A-No. 2099614 describes a digital optical disc reader in which a spot of light is projected onto the disc and light reflected from and scattered by information tracks on the disc is collected by a photodetector. The output of the photodetector is filtered and fed to an analysing circuit including a phase locked loop to provide a high frequency component representing a clock signal for information encoded on the track and a low frequency component indicative of any radial modulation resulting from track wobble.